In cars, which have a longer radius from the axle to the tires' edge, I find a static imbalance to emerge around 45-55 mph.

If I balance said tire and I have a vibration around 65~75 mph, I am suspect of a road force induced imbalance. A road force caused by rim run-out or imperfection in the tires' construction.

Sometimes the road force imperfection can be alleviated by reclocking the tire on the rim 180 degrees. Most of the time it's a runout issue with the rim. ie... bent lip, broken spokes, etc....

The other kick in the nuts is that if you ride a bike with an imbalance for a while, say 500+ miles, you end up wearing the tire circumference into an oblong shape. (Think egg shaped) The oblong shaped tire induces a vibration no matter how many times you put weights on it or put balancing media into it. Dynabeads only work with a good true rim and a new tire. Worn/questionable tires (track take offs) will not always balance out because you don't know if it's worn evenly.

This is why some people bitch that dynabeads didn't work for them. Their used tires are F'ed.